Nowadays, the manufacturers of power supply apparatuses make efforts in developing high quality products. For providing sufficient power energy, more and more power plants are constructed. In addition to the increase of power energy, the power factor and the operating efficiency of electronic devices are important factors for achieving power-saving purposes.
A power factor corrector (PFC) is widely used to compensate the phase difference between current and voltage of an electronic device. Consequently, the input voltage and the input current of the electronic device are in phase, and the power factor is improved. In addition, the use of the power factor corrector can inhibit the harmonic wave that is generated by the electronic device and thus increase the quality of the input power.
FIG. 1 is a schematic circuit diagram illustrating a conventional power supply circuit with a power factor corrector. As shown in FIG. 1, the conventional power supply circuit 1 comprises a power conversion circuit 11, a power factor corrector 12, and a pulse-width modulation control circuit 13. The power conversion circuit 11 is used for receiving AC power from a utility AC power source, and rectifying the AC power into an input voltage Vin. The power factor corrector 12 is used for comparing an output voltage Vo with a reference voltage to perform power factor correction. Consequently, the nearly-sinusoidal shapes of an input current Iin and the input voltage Vin are in phase. In addition, the power factor corrector 12 issues a current control signal Ic to the pulse-width modulation control circuit 13. According to the current control signal Ic, the pulse-width modulation control circuit 13 issues a switch control signal Sc to the power conversion circuit 11. According to the switch control signal Sc, the on/off states of a switching element S of the power conversion circuit 11 are controlled. Consequently, the output voltage Vo is outputted from the power conversion circuit 11.
The power factor corrector 12 may be operated in a discontinuous conduction mode (DCM) or a continuous conduction mode (CCM). By the conventional power factor corrector 12, the input current Iin and the input voltage Vin are in phase, and thus the power factor is enhanced. However, during the transition interval of converting DCM into CCM or converting CCM into DCM, the transient change amount of the current control signal Ic is too large. Correspondingly, the change amount of the duty cycle of the switch control signal Sc from the pulse-width modulation control circuit 13 is too large, and the waveform of the input current Ic is usually suffered from distortion. In particular, during the conversion between DCM and CCM, the waveform of the input current Iin has large total harmonic distortion (THD).
FIG. 2 is a schematic timing waveform diagram of the input current processed by the power supply circuit of FIG. 1. As shown in FIG. 2, at the peak and the trough of the input current Iin, the total harmonic distortion (THD) is very high. For example, the total harmonic distortion factor is up to 6.6 (i.e. iTHD 6.6). Due to the large THD factor of the input current Iin, the quality of the input current is deteriorated.
Therefore, there is a need of providing an improved control method of inhibiting harmonic distortion of an input current in order to avoid the above drawbacks.